A stress sensor has been disclosed in Japanese Unexamined Patent Application Publication No. 2000-267803, in which strain gauges 22 formed by film formation are disposed on a surface of a substrate 20, a post 30 is bonded to another surface of the substrate 20, and the direction and magnitude of a stress applied to the post 30 can be grasped from variation in property of the strain gauges 22 resulting from the application of the stress.
As shown in FIGS. 14(a) and 14(b), the structure comprises: four resistor elements functioning as the strain gauges 22 provided with trimming grooves 21, which are disposed on two lines, being along a surface of the substrate 20 and perpendicularly intersecting each other at the center of the surface of the substrate 20, at substantially the same distance from the center mentioned above; and the post 30 having a square bottom surface bonded so that the center of the bottom surface thereof substantially coincides with the center of the substrate 20 and that each side of an outline 30b of the post bottom surface faces each of the resistor elements 22. In addition, the trimming grooves 21 are formed at two positions of each of the resistor elements 22, the two positions being along each side of the outline 30b of the post bottom surface and provided at the rear side of the substrate 20 corresponding thereto.
In addition, the movement of the stress sensor is shown in FIG. 13(a) in which a stress is applied to the post 30 in an X direction (that is, an optional lateral direction) and in FIG. 13(b) in which a stress is applied to the post 30 in a Z direction (that is, a downward direction).
In the movement of the stress sensor described above, in both cases in which a stress is applied to the post 30 in an X axis or a Y axis direction as shown in FIG. 13(a) and in which a stress is applied to the post 30 in a Z axis direction as shown in FIG. 13(b), solder 32 fixed by a circuit board 31 fixes end portions of the substrate 20, and the stress primarily warps positions of the substrate 20 corresponding to the individual sides of the outline 30b of the post bottom surface. In addition, the structure is formed in which by the stress described above, the strain gauges 22 which are the resistor elements disposed at the positions described above are elongated or contracted.
However, in the case of the structure of the above conventional stress sensor, there has been a problem in that the sensitivity (output) in response to the stress applied to the post 30 is low. It has been believed that the reason for this is that since the stress applied to the post has not been designed to be concentrated on the strain gauges or the design thereof has not been made sufficiently, the stress is likely to be disperses widely over the substrate 20, and as a result, the applied stress has not been effectively used.
Accordingly, a first object that the present invention aims to achieve is to provide a stress sensor having high sensitivity.
In addition, as shown in FIGS. 13(a) and (b), when the operation is performed many times to elongate or contract the resistor elements 22, the elongation or contraction may exceeds the region of elastic deformation in some cases to cause plastic deformation. Due to this plastic deformation, the output resistance from the resistor element 22 in response to subsequent stress application becomes incorrect. The reason for this is that since the plastic deformation is a deformation in which reversibility is lost, the original shape cannot be recovered even when the stress is removed, and a stress resulting from the plastic deformation of the substrate 20 is always applied to the resistor elements disposed on the substrate 20 as described above.
In particular, as shown in FIG. 14(b), when the trimming grooves 21 of the resistor elements 22 are formed along the outline 30b of the post bottom surface, it may be naturally expected that the movement to elongate or contract the resistor elements 22 may cause movement to open and close the trimming grooves 21 as shown in FIGS. 13(a) and (b). In the case described above, it is not too much to say that the plastic deformation of the resistor elements 22 is facilitated. The reason for this is that the trimming groove 21 portions are liable to be plastic-deformed as compared to the other portions. This is because of very large energy which is applied to a resistor forming the resistor element 22 when the trimming grooves 21 are formed.
For example, in the case of laser trimming, the resistor is partly and instantaneously heated to a high temperature, so that the part of the resistor is removed by evaporation thereof. Since this removal process is performed concomitant with large and very rapid change in temperature, of course, cracks may be generated around the periphery of the trimming groove 21 in some cases. The cracks thus generated may widely extend by the movement to open and close the trimming groove 4. As a result, it is expected that the plastic deformation may occur from the cracks as starting sites.
Other trimming methods also partly excavate or damage the resistor forming the resistor element 22 as is the case of the laser trimming. When cracks are generated in the resistor by the reason as described above, a factor serving to embrittle the resistor is additionally generated. As the trimming methods other than laser trimming, for example, sand blasting may be mentioned.
Accordingly, a second object that the present invention aims to achieve is to provide a stress sensor which can achieve the first object and which can maintain the accuracy of output resistance by suppressing the plastic deformation of resistors which are used as strain gauges and form resistor elements provided with trimming grooves.